Low Frequency Oscillator Problem

[Bryan79]

Hi,

I have a question regarding my Crystall Oscillator circuit for low frequency application (32kHz). It is a simple inverter type pierce circuit with ceramic 32kHz model. I manage to run SPICE simulation and the oscillator does start up and oscillate when I ramp up the power supply VDD. The problem is that the frequency of the oscillation is not that one I wanted (32 kHz). At TT (typical corner), it goes to 60 kHz, at SS (slow corner), it goes to 33kHz and at FF (fast corner), it goes to 96 kHz. I couldn't figure out what went wrong, it seems the oscillator can not fix at 32 kHz.

Does anyone have similar experience and any idea how I can solve the problem?

I looked at the Gain and Phase plot, everything looks fine as the Gain is >3 and the phase does crosses 0 degree at around 32 kHz.

Is it overtone thing? But my ceramic model is simple(R1, C0, C1, L1) only. How do I tackle the problem?

Thanks.

 

[Humungus]

COul you post the schematic of your oscillator? I know you said Pierce oscillator, but I would want to verify it.

Also, you may be experiencing relaxation oscillations. That means that your oscillator is working on the charge and discharge of your reactive elements. Is the output signal a sinus or a square wave?